1. Field of the Invention
The present invention relates to a processing system for processing an object to be processed, such as a semiconductor wafer, by a film forming process or the like and, more particularly, to a processing system capable of correcting errors in the position of an object to be processed held by a transfer mechanism.
2. Description of the Related Art
Generally, when fabricating a semiconductor integrated circuit, a surface of a semiconductor wafer, such as a silicon wafer, needs to be repeatedly subjected to film forming processes, oxidation-enhanced diffusion processes, etching processes, annealing processes and the like. To carry out those processes efficiently, a cluster tool system formed by connecting processing vessels for carrying out different processes therein to a common transfer vessel is employed and the wafer is transferred sequentially from one to the other of those vessels via the common transfer vessel to subject the wafer sequentially to the different processes.
Referring to FIG. 10 schematically showing a conventional processing system 2, three processing vessels 8A, 8B and 8C are connected via gate valves 6A, 6B and 6C, respectively, to a common transfer vessel 4. Stages 9A, 9B and 9C each provided on its upper surface with, for example, an electrostatic chuck that attracts and holds a wafer by electrostatic force are placed in the processing vessels 8A to 8C, respectively. Two cassette loading/unloading vessels, i.e., a first cassette loading/unloading vessel 12A and a second cassette loading/unloading vessel 12B, for keeping cassettes containing semiconductor wafers having a shape substantially resembling a disk are connected through gate valves 10A and 10B, respectively, to the common transfer vessel 4. A transfer mechanism 14 including, for example, an articulated arm capable of turning, bending and stretching is installed in the common transfer vessel 4. The transfer mechanism 14 holds a semiconductor wafer W and transfers the same between the cassette loading/unloading vessels 12A and 12B, and the processing vessels 8A to 8C, and between the processing vessels 8A to 8C.
Static electricity must be completely removed from a wafer W before taking the wafer W away from the stage 9A (or 9B or 9C) by the transfer mechanism 14. If the static electricity is not removed properly, it is possible that the static electricity causes the wafer W to jump up from the stage 9A (or 9B or 9C) and dislocate the wafer W and, consequently, the wafer W is held in a dislocated position by the transfer mechanism 14. Therefore, the wafer W must be transferred from the stage 9A (or 9B or 9C) to the transfer mechanism 14 so that the center of the wafer W coincides with the center of a holding device mounted on an end portion of an arm member 16 of the transfer mechanism 14 accurately (within an allowable positional error) scarcely dislocating the wafer W relative to the transfer mechanism 14.
Dislocation sensing devices are disclosed in, for example, JP-A No. 223732/1998 and U.S. Pat. No. 5,483,138. A conventional dislocation sensing device shown in FIG. 10 will be described by way of example. Pairs of linear sensors 18 and 20 spaced a predetermined distance apart from each other are disposed near the gate valves 6A to 6C of the processing vessels 8A to 8C, respectively. When carrying a wafer w past a position corresponding to the pair of linear sensors 18 and 20, the wafer W is stopped temporarily, and two positions of the perimeter of the wafer W are measured. An offset of the center of the wafer W from a reference point is determined on the basis of the measured position of the perimeter of the wafer W.
The pairs of expensive linear sensors 18 and 20, six linear sensors in this processing system, must be installed respectively for the processing vessels 8A to 8C. The expensive linear sensors increase the cost of the processing system. If the wafer W is dislocated by an offset exceeding an allowable limit, the transfer mechanism 14 must be stopped, the common transfer vessel 4 must be opened and a dislocation correcting operation must be carried out. In some cases, a high frequency voltage causes abnormal discharge when a dislocated wafer W is mounted on the stage without correcting the dislocation for a process using a plasma.
The present invention has been made in view of those problems and it is therefore an object of the present invention to provide a processing system capable of determining the dislocation of an object to be processed using a small number of position sensors and of automatically correcting the dislocation of the object.
With the foregoing object in view, according to the present invention, a processing system comprises: a plurality of processing vessels, in which an object to be processed is subjected to predetermined processes, provided with openings, respectively; a common transfer vessel connected to the openings of the processing vessels and internally having stand-by spaces, where the object is held temporarily, determined so as to correspond to the openings of the processing vessels, respectively; a transfer mechanism installed in the common transfer vessel to transfer the object between the common transfer vessel and the processing vessels and capable of turning, bending and stretching; a plurality of pairs of position sensors assigned to the stand-by spaces to measure the positions of the perimeter of the object and arranged so that each stand-by space shares one of the pair of position sensors with the adjacent stand-by space for common use; and a control means that determines an offset of the object held by the transfer on the basis of the position of the perimeter of the object detected by the position sensors and controls the operation of the transfer mechanism so as to correct the offset of the object.
When carrying the object held by the transfer mechanism into one of the processing vessels of the processing system, the object is stopped in the stand-by space corresponding to the processing vessel. The pair of sensors assigned to the stand-by space measure the positions of two points on the perimeter of the object. The control means determines an offset of the object held by the transfer mechanism on the basis of the respective positions of the two points on the perimeter of the object and controls the operation of the transfer mechanism so as to correct the offset of the object. Consequently, the object held by the transfer mechanism can be correctly placed at a predetermined position in the processing vessel, such as a predetermined position of a stage placed in the processing vessel.
Each stand-by space shares one of the pair of position sensors with the adjacent stand-by space for common use, and hence the number of the necessary position sensors is reduced accordingly. The reduction of the number of the necessary position sensors reduces the manufacturing cost of the processing system effectively.
The stand-by spaces may be arranged about the center of turning of the transfer mechanism at angular intervals of, for example, about 60xc2x0.
Preferably, the position sensors are linear optical sensors each having a photodetector of a predetermined length, and each position sensor is disposed so that the photodetector is aligned with a line passing the center of the object accurately placed in the corresponding stand-by space.
Preferably, the control means stops the operation of the transfer mechanism when the offset of the object is not smaller than a predetermined value.
The operation of the transfer mechanism is stopped when the offset is not smaller than the correctable limit to prevent the collision of the object against side wall of the processing vessel or the like with reliability.
According to the present invention, another processing system comprises: a processing vessel, in which an object to be processed is subjected to predetermined processes, provided with an opening; a transfer vessel connected to the opening of the processing vessels and internally having first and second stand-by spaces having centers spaced a predetermined angular interval apart, where an object to be processed is held temporarily; a transfer mechanism installed in the transfer vessel to transfer an object to be processed between the transfer vessel and the processing vessels, capable of turning, bending and stretching and having a center of turning about which the first and second stand-by spaces are arranged at an angular interval; a position sensor disposed so as to extend over the first and second stand-by spaces to measure the position of the perimeter of the object; and a control means that carries out a control procedure including the steps of determining the position of the perimeter of the object held by the transfer mechanism and located in the first stand-by space by the position sensor, making the transfer mechanism turn through an angle equal to the angular interval between the first and the second stand-by space to locate the object in the second stand-by space, determining the position of the perimeter of the object located in the second stand-by space by the position sensor, determining an offset of the object on the basis of the positions of the perimeter of the object as located in the first and the second stand-by space detected by the position sensor, and controlling the operation of the transfer mechanism so as to correct the offset of the object.
The object held by the transfer mechanism is stopped in the first stand-by space before being carried into the processing vessel and the position of the perimeter of the object is detected by the position sensor. Then, the transfer mechanism is turned through the angle corresponding to the predetermined angular interval between the first and the second stand-by space to move the object to the second stand-by space. The sensor detects the position of the perimeter of the object held in the second stand-by space. The control means determines an offset of the on the basis of two positions of the perimeter of the object detected by the position sensor and controls the operation of the transfer mechanism so as to correct the offset of the object. Consequently, the object held by the transfer mechanism can be carried to and accurately placed at predetermined position in the processing vessel, such as a proper position on a stage placed in the processing vessel.
Preferably, the position sensor is a linear optical sensor a photodetector of a predetermined length, and the position sensor is disposed so that the photodetector is aligned with a line passing the center of the object accurately placed in either of the first and the second stand-by space.
Preferably, the control means stops the operation of the transfer mechanism when the offset of the object is not smaller than a predetermined value.